1. Field of the Invention
The present invention relates to a time-synchronous communication system which is designed to be able to easily and efficiently perform circuit connection and a receiving/processing operation which are performed when bidirectional communication between mobile radio stations such as ships or automobiles scattered in a large area and a terrestrial fixed radio station is performed by using a plurality of satellites as radio wave relay means.
The present invention makes it possible to construct an efficient communication system for bidirectional message communication among automobiles, ships, airplanes, and the like which are scattered in a large area. More specifically, the efficient communication control required when satellites are used as relay stations is realized. The realization of a communication circuit control scheme on the assumption that time synchronization is established as the base of the present invention becomes high with development of a recent precise clock technology and a precise distance measuring technology.
With development of the satellite radio communication technology, satellite navigation or satellite communication are included in a public and general engineering field. For example, when a distance measuring radio wave from a GPS (Global Positioning System) satellite is received and processed, high-precision position measurement of a mobile station or the like and high-precision synchronization of a time reference can be performed. Therefore, in the fields of traffic control, operation management, search and rescue, and the like of mobile vehicles, a satellite position-measuring system such as the GPS is used as a matter of course, a position-measuring means and a time-synchronization means can be easily obtained.
On the other hand, as a radio communication means for a mobile vehicles, a bidirectional communication means such as a portable telephone or a personal radio is known. However, a proper mobile radio communication means which can be used at low cost in a large area including the ocean has not developed. The present invention makes it possible to extremely easily and theoretically perform mobile radio communication which can be used in a large area including the ocean. This mobile radio communication can be early realized on the field of international electric communication industries.
2. Description of the Related Art
A related art concerning the present invention will be described below. Of various communication systems, a communication system in which the transmission side spreads the spectrum of a transmission radio wave by the code of a pseudo-random noise (to be referred to as PRN hereinafter) sequence to transmit the radio wave and the reception side inversely spreads the spectrum of the reception radio wave by the same code to decode the radio wave has been widely used as a system having high resistance to radio interference or concealment, and the above communication system is widely used as a code-division multiple access system (to be referred to as CDMA hereinafter) in a circuit connection method. In addition, a communication system in which a radio wave is transmitted within an assigned time slot, and radio waves having the same frequency are simultaneously used as carrier waves by a plurality of users is a mature technology as a time-division multiple access system (to be referred to as TDMA hereinafter).
Radio communication uses a principle that a radio wave having a certain frequency is used as a carrier wave, and the carrier wave is modulated by serial data to be transmitted. Therefore, on the reception side, the head of the serial data must be specified. However, in the CDMA, since the carrier wave is modulated by the PRN sequence code having a frequency higher than that of the serial data, the signal before spreading must be reproduced by inverse spectrum spreading. When this inverse spreading is to be performed, the inverse spreading must be synchronized with the timing on the spectrum spreading of transmission side. For this reason, in a GPS developed in the U.S.A, the spectra of radio waves synchronously transmitted from a plurality of satellites are inversely spread, and, by using a delay time extracted in the inverse spreading, the difference between arrival times of a plurality of radio waves transmitted from the plurality of satellites is calculated to estimate the position of a reception point.
In this manner, the CDMA technique is used in not only the field of communication which requires resistance to interference and concealment but also the field of measurement technique which requires precise time-synchronous conditions between the transmission side and the reception side. However, it is generally said that the CDMA uses a frequency at low efficiency, and the CDMA is slightly suitable for mobile communication for a large number of communication users.
On the other hand, the TDMA is a multiple access system which is effective when the transmission rate of one communication circuit is considerably higher than a transmission rate required on the transmission side, and is a communication system in which one communication circuit is divided into a plurality of time slots to be assigned to a plurality of communication users, so that the same communication circuit is simultaneously shared by the plurality of users. For example, assume that a plurality of mobile stations are set as a transmission side, and that one fixed station is set as a reception side. In this case, when the fixed station provides timing information for specifying a time slot to be used to each mobile station, radio waves can be continuously received in time series without congesting the radio waves from the plurality of mobile stations. Therefore a bidirectional mobile communication system can be constructed when data is transmitted from the fixed station to the plurality of mobile stations in a time divisional manner.
Consequently, the TDMA is effective as a method of efficiently using a communication circuit having a high transmission rate by a plurality of users. However, when one radio circuit is used by a large number of mobile stations, there are a large number of problems to be considered in circuit control such as setting of guard time, assignment of circuits, and synchronization of communication timing.
Although the communication technique such as the CDMA or the TDMA has the above problems, these techniques can also be easily used in space communication. Especially, the CDMA is expected to be frequently used in a telephone circuit using a satellite in the near feature because of its excellent concealment or resistance to interference. However, although an amount of data transmitted at a time from each mobile station is small, when a message communication system which can be used at low cost is constituted on the assumption that a very large number of mobile stations are simultaneously used, the codes of a large number of PRN sequence whose low correlation properties in the CDMA are prepared, and these codes must be assigned to the respective users. Therefore, the communication techniques have a large number of problems on the cost of apparatuses and the facilities of operation.
In case of message communication, since a period of circuit connection time of one mobile station is relatively short, the efficiency of use of circuit is degraded in a conventional demand assignment scheme for assigning a radio circuit in correspondence with a request from a mobile station. In particular, this problem conspicuously appears in case of the TDMA system. More specifically, in order to avoid congestion of circuits, circuit connection is generally required by a mobile station first, and a fixed station (terrestrial station) performs circuit assignment. However, in message communication having a small amount of information transmission, a period of communication time for circuit connection occupies a large part of a total period of communication time.
In mobile communication using a satellite as a relay station, mobile stations are scattered in a large area, an arrival time of a mobile transmission radio wave reaching a fixed station through a satellite is dispersed depending on different radio wave propagation distances. Therefore, in order to prevent interference of communication circuits, a guard time between time slots must be set to be large. In this manner, the conventional connection scheme of communication circuits is not completely efficient for message communication using a satellite.
Bidirectional communication using a satellite employs a multiple access system in which inquiry (polling) signals are sequentially transmitted from a fixed station to all mobile users, and the users respond to the polling signals. However, this system cannot be called a communication system in which a large number of users who want to arbitrarily make communication as needed can be accommodated.
Another problem of mobile satellite communication using a satellite as a relay station is posed when a plurality of satellites are arranged in the sky. Since the antenna of a mobile station is generally omni-directional, a radio wave transmitted from the mobile station is received and relayed by all the satellites arranged in the sky and then transmitted to the terrestrial fixed station. Therefore, all radio waves reaching the fixed station through a plurality of satellites are received, the receiving/processing apparatus of the fixed station increases in scale, and processing of the received radio waves becomes severe. This is because transmission radio waves of all the mobile stations are extracted from asynchronous radio waves reaching the fixed station at various timings in a hybrid manner. When mobile stations or fixed stations increase in number, degradation of operation efficiency of the communication system caused by the above problem is expected to be more serious.
As described above, the code-division multiple access system (CDMA) using spectrum spreading modulation is used as a military communication having strong to radio wave interference or a space communication system capable of demodulating faint radio waves, and the time-division multiple access system (TDMA) is used together with a frequency-division multiple access system (FDMA) as a technique which has been mature in terrestrial communication and satellite communication. However, even if these known communication systems are combined to each other as a multiple access system for bidirectional message communication using a plurality of satellites and a large number of fixed stations as radio base stations, an efficient satellite communication system cannot be obtained.